Hydraulic pistons are used for force application in various places in different types of automatic transmissions. In some instances, such pistons are mounted on a servo pin that extends from one end of the piston. The servo pin typically slides in a bore formed in a part of the transmission housing. In some cases, high loads or excentric loading on the piston may create wear and scoring in the pin bore or on the pin itself, which can restrict the motion of the piston. This in turn can cause damage to the piston and limit its effectiveness. Excentric loading can also contribute to increased friction acting on the piston as it moves.
One exemplary automatic transmission that employs a piston and servo pin of the type described is the Powerglide transmission. FIG. 11 illustrates a cross-sectional view of a conventional Aluminum Powerglide transmission piston assembly 10 with piston 16 and servo pin 18. In this application, the piston and servo pin are used to apply a band clutch (not shown) via hydraulic pressure of the transmission. Hydraulic pressure from the valve body (not shown) is applied through a hole (not shown) in the housing 15 and piston bore 12 to the apply side 24 of the piston, moving the piston and pin down the bore, applying force to the band clutch. The piston is supported on the servo pin, which slides in servo pin bore 14. The band clutch is released by spring force and fluid pressure delivered to the release side 26. The cover 20 is fixed to the piston bore and sealed by cover seal 22. During operation, as mentioned above, the servo pin may bind and not move freely. When this occurs, the piston/pin may not retract completely, leaving the band clutch partially engaged and dragging, which can severely damage the band clutch lining and clutch drum requiring repair. Investigation of a defective prior art servo pin revealed evidence of scoring marks 28 (see FIG. 12) on the outer diameter of piston 16 due to excessive contact with the piston bore.
Previously, such defects and wear have been addressable only by “after-the-fact” solutions that involve reconditioning the worn pin bore. One such prior reconditioning solution is provided by the present Applicant's prior “Servo-Fix” kit, which includes a jig for accurate re-boring of the servo pin bore and installation of a repair sleeve. A bore reconditioning approach to this type of problem is also described in U.S. Pat. No. 7,771,144 entitled “Universal Jig/Work Holding Fixture and Method of Use.” Thus, while repair solutions have existed to recondition a servo pin and its bore after it has become damaged, there has remained a need in the art for a solution that prevents the damage before it occurs.
Note that as used herein, “Aluminum Powerglide”, “Powerglide” or “Powerglide transmission” refers not only to Powerglide transmissions and parts originally manufactured by General Motors, but also to any aftermarket transmissions or parts substantially conforming to the original dimensions and specifications.